The present invention relates to band pass filtering of electrical h.f. signals by means of solid state tuned devices. More particularly, the present invention relates to improvements in the art of band pass filtering using filter elements which include bodies of piezoelectric material, such a body being provided with electrodes for producing and responding to acoustic surface waves. Filter devices of this type are also called surfaceacoustic wave filters or SAW-filters for short.
A typical device of the character referred to above includes a more or less thin wafer of a piezoelectric material. Quartz is one of the "classical" piezoelectric substances; however, more recently Lithium-niobate crystals have been used because such a material has better properties, for example, as far as frequency selectivity is concerned. Such a wafer is provided on its surface with a first array of equidistantly spaced metal strips or fingers, which are interconnected by two transverse strips in that the fingers are alternately connected to these transverse strips to establish two interleaved or interdigitized combs. A second such array is provided on the same surface of the wafer, but being spaced from the first array in a direction transverse to the extension of the fingers; the fingers of both arrays extend in parallel to each other.
The metal fingers in the several arrays coact as electrodes with the piezoelectric material underneath, and each pair of interdigitized combs having these fingers establishes an electro-acoustic transducer. As an electrical signal is applied between the combs of one of the transducers, surface waves are set up and propagate to the other array (transducer), which develops a voltage between its combs. Such a device has an inherent frequency response (pulse response) which is centered at a frequency determined by the center-to-center spacing of the interleaved fingers of the transducers.
A simple comb pattern for each of the arrays produces a frequency response which has also significant side lobes. Thus, such a pair of transducers which are coupled acoustically by the piezoelectric wafer have multiple transmission bands. Through various techniques, not ably the so-called apodization, these side lobes can be suppressed leaving only a pronounced pass band. Apodization involves primarily contouring the overlap of the interdigitized fingers of the combs. The pattern determines and, actually, synthetizes the desired response, so that the off-band side lobes be suppressed.
Another aspect with regard to pass band synthesis is the use of a three transducer system, a central one and two outer ones on the same piezoelectric wafer. The primary purpose of using a three transducer system is to increase the overall gain of such a device. The finger spacing of the outer ones may differ slightly from the finger spacing of the central transducer to widen the pass band. The outer transducers may be operated as transmitters which are driven electrically in parallel, and the center transducer may receive acoustic waves from both transmitters to establish a single output. Conversely, the centrally positioned transducer acts as transmitter, which launches surface waves in opposite direction, and these surface waves are picked up by the two outer transducers.
Response and pass band synthesis has been quite successfully achieved in the past by means of these devices. However, they exhibit quite frequently rather disturbing side effects. The transducers as described do not just produce surface waves (Rayleigh waves) in the piezoelectric body, but compression and shear waves, also termed bulk waves, propagate into the interior of the piezoelectric body. They are reflected on the underside and reappear at the electrode bearing side of the body. Some of these reflected waves will reach the pick-up transducers and the latter will respond. It can readily be seen that some of these waves bounce back and forth between the opposite surfaces of the wafer. Such multiple reflection will cause significant attenuation, however, bulk waves which have been reflected but once will have significant strength when reaching a receiving transducer. These bulk waves cause deterioration of the off band rejection properties of the tuned device.
The generation of the bulk waves is a process quite different from the generation of a surface wave whose frequency is based on particular spacing of the fingers. Each finger produces also an incremental wave that propagates into the wafer, and the frequency of these waves has little relation to the finger spacing. As a consequence, the transmitting transducer produces waves outside of the desired pass band. These parasitic, spurious, off band side lobes in the frequency response may be only about 20 db, or even less, below the frequency response of the pass band. The acoustic transmission of a pass band signal is, therefore, interfered with by a dual transmission; a transmission via bulk waves is superimposed upon the transmission via surface waves.
A somewhat related problem is posed by the so-called plate modes of a rather high order. They are stimulated by the transmitting transducer(s) and are inevitably effective in the pick-up or receiving transducer(s). These plate modes contribute to the problem particularly if involving the same frequencies in the response of the device to bulk waves. In order to facilitate the present description, I shall include the plate modes in the bulk waves.
Another vexing problem of these solid state acoustic devices are the so-called multiple transit echos, notably the triple transit echo. This phenomenon is based on the reversibility of the interaction between the electrodes and the piezoelectric material; or, to say it differently, each transducer can act as transmitter and as receiver. Thus, a receiving transducer when stimulated will act as a retransmitting, echo-producing device. That echo is picked up by the transmitter proper and retransmitted again to be picked up by the receiver etc. Of course, the signal loses strength on each retransmission, but a signal that was bounced twice back and forth has strength only, say, twelve db down from the signal as it was picked up first. Since the echo has pass band frequency, it cannot be attenuated by frequency selectivity.